In modern society, bath tissue, paper towels, facial tissue, and paper napkins (hereinafter referred to as packaged paper products) have been remarkably successfully consumer products. The success of these paper products stems from the ability of manufacturers to consistently enhance product attributes at lower cost and to meet volume demands on a timely basis. Packaged paper products offer consumers an array of attributes necessary to such jobs as performing the daily tasks of wiping up spills, personal cleansing, and cleaning household goods. For example, paper towels are engineered to be absorbent and strong while wet whereas bath tissue products are expected to be soft to the touch yet strong while in use. Absorbency and softness are inversely related to strength, often making it difficult to obtain the right balance of attributes. Accordingly, significant research and development efforts are routinely expended to enhance the quality of these products while continuing to reduce cost by, for example, improving the production of these products. Although numerous schemes have been developed and patented, the search by R&D departments continues to seek out new and innovative methods for improving these products.
There are numerous methods described in the patent literature for improving the quality of packaged paper products. One of the earliest known methods to enhance the quality of consumer paper products is described in U.S. Pat. No. 3,301,746 by Sanford and Sisson, assigned to Procter and Gamble Corporation, and incorporated herein by reference in its entirety. This patent describes a papermaking scheme for enhancing product quality by avoiding overall web compression and by using a pattern array of densified regions in the xy plane of the sheet to enhance product strength.
Other early methods for improving the quality of packaged paper products are described in U.S. Pat. No. 3,812,000 by Salvucci and Yiannos and U.S. Pat. No. 3,821,068 by Shaw. These patents are assigned to Scott Paper Company, each of which is incorporated herein by reference in its entirety. Shaw discloses a papermaking scheme for producing soft tissue by avoiding mechanical compression until the sheet has been dried to at least 80% solids. Salvucci and Yiannos disclose a technique for producing a soft tissue structure by avoiding mechanical compression of an elastomeric containing fiber furnish until the consistency of the web is at least 80% solids.
Thicker more absorbent structures can be made using a low batting papermaking felt as described in U.S. Pat. No. 4,533,457 by Curran et al., assigned to Scott Paper Company, and incorporated herein by reference in its entirety. U.S. Pat. Nos. 5,591,305 and 5,569,358 by Cameron, assigned to James River Corporation, and incorporated herein by reference in their entirety, disclose a lowbatting, high-bulk-generating felt with improved dewatering capabilities.
A more recent method for improving the quality of a through-air-dried sheet is described in U.S. Pat. No. 4,440,597 by Wells and Hensler, assigned to Procter and Gamble Company, and incorporated herein by reference in its entirety. This patent describes a method for increasing the stretch of a paper web by operating the forming section of a paper machine faster than the through air dryer section of the paper machine. As a result of the speed differential, the paper web is inundated into the through air-dryer-fabric leading to enhanced stretch and absorbency properties in the base sheet and resulting product.
Fibers and chemicals can be used to enhance the quality of packaged paper products. For example, U.S. Pat. No. 5,320,710 by Reeves et al., assigned to Fort James Corporation, and incorporated herein by reference in its entirety, describes a new furnish combination extracted from the species Funifera of the genus Hesporaloe in the Agavaceae family. This furnish has fibers which are very long and which have very fine-geometrical attributes known to enhance tissue and towel performance. U.S. Pat. No. 3,755,220 by Freimark and Schaftlein, assigned to Scott Paper Company, and incorporated herein by reference in its entirety, describes a debonding scheme for maintaining wet strength while reducing product dry strength-a method known to enhance the handfeel of towel products.
The use of bulking fibers can improve the quality of the final end product. U.S. Pat. No. 3,434,918 by Bernardin, U.S. Pat. No. 4,204,504 by Lesas et al., U.S. Pat. No. 4,431,481 by Drach et al., U.S. Pat. No. 3,819,470 by Shaw et al., and U.S. Pat. No. 5,087,324 by Awofeso et al. disclose the use of bulking fibers in papermaking webs to improve product attributes like thickness, absorbency, and softness. These aforementioned patents are incorporated herein by reference in their entirety.
U.S. Pat. No. 5,348,620 by Hermans et al., assigned to Kimberly-Clark Worldwide Inc., and incorporated herein by reference discusses a high consistency/high temperature fiber-treatment-process using a disperser to improve product attributes. To improve tissue softness, several approaches are available to the papermaker such as using certain species of hardwood like eucalyptus in stratified webs as disclosed in U.S. Pat. No. 4,300,981 by Carstens and U.S. Pat. No. 3,994,771 by Morgan et al. The last two patents are incorporated herein by reference in their entirety. These aforementioned patents describe just a few of the many methods developed over the last thirty years to enhance the quality of packaged paper products.
There are also numerous schemes for enhancing the productivity of paper machines. For example, gap formers have been developed to enhance sheet drainage ultimately leading to increased machine speed. New developments in Yankee hood design and Yankee cylinder design have allowed improvements in heat transfer coefficients and mass transfer coefficients, ultimately leading to enhanced machine speeds. New developments in forming fabrics, e.g., multi-layer and triple-layer forming fabrics, have resulted in improved drainage, better fabric life, and enhanced fiber support. These factors translate into enhanced machine speed and productivity. Improvements in press felts, e.g. Scapa's SPECTRA .TM. felt concept of using a soft polyurethane sandwich near the base of the felt or the use of stratified batting, have led to improvements in felt life, reductions in break-in time, and improvements in water removal at wet presses. These improved press-felt developments have ultimately translated into improved machine speed and productivity. Improvements in Yankee creping adhesives have been helpful to enhance blade wear and reduce sheet plugging. Continuous creping doctors have alleviated the need to frequently change doctor blades. The last two aforementioned developments have led to improvements in machine speed, reductions in down time, and reductions in paper waste. In spite of all these advances, methods are sought to enhance productivity.
The present invention improves the efficiency of known water removal methods by adding one or more pressing units to the production paper machine, in place of or in conjunction with a suction pressure roll. "Pressing units" according to the present invention include those units that physically engage a belt or pressing blanket, which contacts the impression fabric or felt upon which the web travels. "Foraminous endless fabric" as defined in accordance with the present invention includes either an impression fabric or felt. "Pressing unit" as defined in accordance with the present invention includes any press members allowing deformation of the pressing blanket/impression fabric and/or felt/web sandwich to result in asymmetric pressure profiles. These pressing units including pressing blankets are generally discussed in the literature as "shoe presses." Pressing units according to the present invention do not include suction pressure rolls since they lead to symmetrical pressure distributions frequently mathematically described by sine or haversine functions.
Shoe presses have been used to increase water removal at wet presses, ultimately leading to increased machine speed for linerboard grades and more recently, newsprint and fine paper grades. The idea of extending the time in a press nip as a means to enhance water removal is not a new idea. Nissan in 1954 published a paper in Tappi, Vol. 37, No.12, p.597 (1954) suggesting that the use of extended time in a press nip would enhance the water removal performance of a press. Over, twenty-five years ago Busker published an early paper in Tappi, Vol. 54, No.3, p.373 (1971) on the use of extended nip times, as a means to enhance water removal. Beloit Corporation commercialized the first open belt wide shoe press on a linerboard machine in 1980 as described in an article by J. Blackledge presented during the 2.sup.nd International Pira Conference, entitled `Modern Technologies in Pressing and Drying`, Nov. 6-8, 1990, p. 1. The aforementioned three articles are herein incorporated by reference in their entirety.
FIG. 1 shows a typical closed belt wide shoe press (see FIG. 2 in an article entitled "New Pressing Technologies for Multiply Board" by J. Breiten in 81.sup.st Annual Meeting, Technical Section, CPPA, p. A137 for a more detailed drawing). A wide shoe press as described in the literature is essentially a controlled crown roll with a flexible shell and a concave shoe hydrodynamically loaded against each other. The belt or blanket is usually a fabric reinforced polyurethane-coated structure that can be grooved or blind drilled for more efficient water removal. The inside of the belt is generally lubricated with oil, which develops a hydrodynamic film as it passes over the shoe and reduces wear/friction in both surfaces. Wide shoe press nips are on average 5 to 10 times longer than conventional roll press nips (generally, 5"-10" versus 1"-2"). Water deflectors (not shown) on the outside surface will dewater the blanket. By utilizing such a wide nip, loads up to 10,000 pli are possible without the risk of damaging blankets and felts or crushing the sheet. The exit side of the shoe features a sharply curved nose designed to pull the sheet directly out of the nip and away from the felt, thus reducing rewet and improving sheet dryness. U.S. Pat. No. 4,931,142 describes certain advantages to this type of take off angle in conjunction with long press nips. Rolls do not normally support the belt loop of the wide shoe press. The loop generally is closed off with special head assemblies for containing the oil.
Numerous schemes for improving the operation of shoe presses have been developed over the years. For example, in U.S. Pat. No. 5,043,046 by Laapofti and assigned to Valmet Corporation, U.S. Pat. No. 4,625,376 by Schiel et al. and assigned to Voith Corporation, and U.S. Pat. No. 4,673,461 by Roerig and assigned to Beloit Corporation, methods are described to enclose the shoe press in order to contain the oil within the unit. The previous three patents are incorporated herein by reference. U.S. Pat. No. 5,167,768 by Cronin and Roerig and assigned to Beloit Corporation and U.S. Pat. No. 5,582,689 by Rolf Van Haag and Hans-Rolf Conard and assigned to Voith Corporation describe methods for varying the pressure distribution in a shoe press. This capability avoids the need to offset the center of loading or reshape the shoe to change the pressure distribution. These last two patents are also incorporated herein by reference. U.S. Pat. No. 5,693,186 by Vallius, assigned to Valmet Corporation, and incorporated herein by reference describes a tension link scheme for containing the loading within the framing of the shoe press apparatus. This scheme ultimately avoids the need to fortify flooring when operating at high line loads. These are just a few of the many developments that have led to improved operating shoe presses.
In the art of pressing linerboard, newsprint, and fine paper webs with a shoe press, a long shoe with a gradual pressure rise is desirable for good dewatering and enhanced bulk properties. This is especially true for flow controlled webs. Linerboard and to a certain extent newsprint and fine paper have flow controlled pressing conditions. Flow controlled pressing conditions occur when the time in the nip becomes an important factor determining the amount of water removed from the web. High pressure can be attained with these long shoes but it requires high line loads. FIG. 2 shows the relationship between peak pressure (i.e., the maximum pressure in the nip) and line load (i.e., the total force divided by linear width) for shoe press nips compiled from an extensive but not exhaustive search of the literature. Table I describes the literature references used to develop FIG. 2.
TABLE I References Used to Generate Figure 2. Reference Number Source 1 U.S. Pat. No. 5,167,768 2 W. Schuwerk, Paper Age, September, 1997, p. 18. 3 N. Anderson, Journal of Tappik, Vol. 21, No. 1, 1998, p. 52. 4 J. Kinnunen and A. Kiviranta, Paperi Ja Puu-Paper and Timber Vol. 74, No. 4, 1992, p. 314. 5 J. Kivimaa, M. Laurikainen, and K. Pansu, PITA Water Removal Conference 1997 York, Paper Technology, April, 1998. 6 J. Blacklege and D. Lange, 2.sup.nd International Pira Conference, "Modern Technologies in Pressing and Drying", Nov. 6-8, 1990, p. 1. 7 M. Radtke, 79.sup.th Annual Meeting, Technical Section, CPPA, p. A221. 8 J. Breiten, 81.sup.st Annual Meeting, Technical Section, CPPA, p. A137 9 E. Tenfalt, J. Wilmenius, and O. Swanberg, Nordic Pulp and Paper Research Journal, 1998, p. 16. 10 D. Lange and M. Radtke, Papermaker, July 1996, p. 16. 11 "Chemical Systems Boost Dry Content", PPI, February, 1989, p. 41.
The graph in FIG. 2 shows that shoe presses normally operate at high line load conditions, usually greater than 270 kN/m and at high peak pressures. It also shows that shoe presses are not operated at low line loads and at high peak pressures (e.g., see the crosshatched region in FIG. 2).
In the art of making tissue by the conventional wet pressing operation, Yankee dryers are loaded with suction pressure rolls to remove water from the tissue web and attach the web to the dryer for further processing by the creping operation. The pressure distribution in the suction pressure roll nip is symmetrical in shape and is described mathematically by a sine or a haversine curve. Suction pressure rolls loaded to a Yankee dryer are routinely run at line loads less than 100 kN/m and at peak pressures of less than 4500 kN/m.sup.2. In the lower right-hand corner of FIG. 2 some typical peak pressure versus line load data for suction pressure rolls are shown. The deflection of large, conventional Yankee dryers due to hoop stress levels limits the line load to less than about 100 kN/m. As a result, it is very difficult to attain high peak pressures in the nip at these low line loads, since the pressure distribution cannot be altered. This limitation has extreme consequences for tissue grades since they are pressure controlled, i.e., the flow resistance in the web is low due to the use of high freeness furnishes and low basis weight webs, thus it is believed that peak pressure, not time in the nip, controls press dewatering. These suction pressure rolls suffer from other disadvantages. For example, since the nip diverges after the maximum pressure is achieved, rewet can occur for a large part of the press nip. A typical suction pressure roll curve appears in FIG. 3, where nip divergence is apparent. Also, the suction pressure roll unit is not flexible so that the line load needs to be fixed and matched to a given Yankee crown condition in order to obtain a uniform nip profile across the machine. Furthermore, since the loading cylinders are located at each end of the pressure roll, profiling capabilities are very limited.
The use of conventional shoe presses on a Yankee dryer at the maximum hoop stress limit of 100 kN/m would lead to very low peak pressures as FIGS. 2 and 3 demonstrate. For example, with a 120 mm shoe at 100 kN/m, the typical peak pressure is on the order of 1700 kN/m as FIG. 3 demonstrates. Since the press nip for low weight tissue and towel grades is pressure controlled, the very low peak pressure could cause a decrease in post press dryness, ultimately causing a loss in production. The counter roll in a conventional shoe press is small by comparison to the diameter of a Yankee dryer. As a result, the use of a conventional shoe shape would make it very difficult to remove the felt/fabric from the sheet at the nip exit. Therefore, conventional shoe shapes and conventional felt/fabric takeoff angles would exacerbate rewet for low weight absorbent products.
Currently, there are no commercial uses of shoe press technology in the production of absorbent paper products. U.S. Pat. No. 5,795,440 by Ampulski et al., and U.S. Pat. No. 5,776,307 by Ampulski et al.-both assigned to Procter and Gamble Corporation and both incorporated herein by reference, describe a scheme for making a shaped web by pressing an embryonic web into an imprinting fabric between two felts. These patents use a shoe press assembly in the preparation of a wet pressed paper web. Ampulski et al., like others using pressing units, teaches the use of longer conventional press nips. Ampulski et al. discloses that the nip length is greater than 3.0 inches and may be as long as 20.0 inches. Ampulski et al. achieves this extended nip length through the use of a shoe press. Ampulski et al., like all previous users of shoe presses, fails to consider the use of increased peak pressure.
International patent application WO 97/43483 by Hermans and Friedbaurer, assigned to Kimberly-Clark Worldwide, Inc., and incorporated herein by reference discloses that extended nip presses, while having been successfully used for making paperboard, have not been used to make low density paper products such as tissue because the high pressure and longer dwell times in an extended nip press serve to densify the sheet beyond that experienced by conventional tissue wet pressing methods. Hermans and Friedbaurer overcome the increased density due to extended nip pressing by incorporating modified resilient fibers (e.g., chemically cross-linked cellulosic fibers) in the web and by wet micro-shaping the web. They also disclose shoe lengths typically in the range of 5 to 10 inches. Like Ampulski et al., Hermans and Friedbaurer do not consider critical peak pressures or line loads as important.
U.S. Pat. No. 5,393,384 by Steiner et al., and assigned to J. M. Voith, GmbH (hereinafter "the '384 patent") generally describes the use of a shoe press in the production of a tissue web. The '384 patent describes the use of a shoe press preceding or contacting a Yankee drying cylinder. The shoe press is used in conjunction with an impermeable belt to reduce remoistening of the sheet by the felt. These authors used the impermeable belt since they state: "the prevailing opinion in selecting suitable drying presses in contingence on the web thickness so far has been that for drying thin webs there are only simple roll presses suited which generate a sufficiently high contact pressure for a short time, thus optimally removing the water from a thin web (tissue web) due to the short path, whereas shoe type presses are suited essentially for drying thick, heavy webs, since they generate a persistent pressure which allows the water sufficient time for the considerable longer path in leaving the web." Critical peak pressure and line loads are not discussed in the disclosure. Since the shoe press described in this disclosure is conventional, a pressure curve for this type of shoe press is most likely similar to the "typical shoe press curve" illustrated in FIG. 3.
Voith, the assignee of the '384 patent, continues to develop the use of a shoe press for the production of paper products. U.S. Pat. No. 5,500,092 by Schiel describes a tissue making machine using a triple press nip where the second nip is a shoe press nip. The criticality of pressure distribution shape and peak pressure/line load magnitudes are not disclosed in the '092 patent. In the September 1997 article W. Schuwerk, "Shoe Presses and Sleeves for Newsprint-Concepts and Initial Operating Experience," PaperAge, Pp. 18-23, Voith described the advantages of their NIPCOFLEX shoe press. According to that article, "[T]o obtain optimum product characteristics, dewatering in the press must [therefore] show as flat a pressure gradient as possible." In fact, the shoe press described in the article refers to the third section of a newsprint paper machine operating at a line loading of 850 kN/m and a peak pressure of .about.5.6 MPa, typical of standard conventional shoe designs and well outside the range of the present invention.
U.S. Pat. No. 4,931,142 by Steiner, Muller, Schiel, and Flamig, assigned to Voith Corporation and incorporated herein by reference in its entirety describes a method of eccentrically arranging a press blanket with respect to the press plane. This arrangement enables the blanket upon leaving the press nip to immediately move steeply downward and away from the sheet in order to reduce remoistening of the web. Methods of varying the felt angle with respect to the traveling web in a double felted press nip were disclosed to avoid remoistening the sheet and for quick release of the sheet from the felt. Steiner et al. also discloses that the joint path of travel of the paper web, felt, and blanket can be made substantially shorter than prior art. By utilizing the Steiner et al. invention, the joint travel of the felt, web, and blanket can be made equal to zero, i.e., the web can detach itself from the felt directly at the emergence from the press nip. Steiner et al. does not address low line loads and high peak pressures needed for optimum shoe press performance on Yankee dryers. It also does not disclose the need to taper the press shoe to achieve minimized rewet.
U.S. Pat. No. 5,556,511 by Bluhm and Gotz, assigned to Sulzer-Escher Wyss, and incorporated herein by reference describes a process for making toilet tissue webs whereby a web is wet pressed in a heated pressing arrangement. The heated pressing arrangement can be a shoe press. This disclosure does not address the importance of proper choice of peak pressure, line load, and shoe shape for making absorbent products at high speeds. In fact, the critically of line loads and peak pressures is not discussed. Bluhm and Gotz like all previous users of shoe presses, fails to consider the use of increased peak pressure at low line loads as a means to improve water removal.
U.S. Pat. No. 4,973,384 by Crouse, Pulkowski, and Porter, assigned to Beloit Corporation, and incorporated herein by reference describes a process for using a heated extended nip press for optimizing sheet properties without lamination. To accomplish the aforementioned task Crouse et al. found that by application of pressure for an increased period of time, the increased residence time enables the removal of more water from the formed web. As a result, these authors teach toward the use of a conventional long shoe design. They also found that for a heated extended nip press by "gradually decreasing pressure in machine direction toward the trailing edge of the shoe, rapid flashing of steam from the emerging pressed web was avoided." As a result these authors teach away from the use of a heavy peaked pressure distribution at the exit side of a shoe press nip.
WO 97/16593 by Wedel and Worcester incorporated herein by reference discloses an impulse drying method for tissue structures using a shoe press and an induction heater. This disclosed impulse-drying method is intended to replace the Yankee dryer with its associated problems. These authors list the issues with Yankee dryers as being limited in surface temperature to 185.degree. F., as being limited in line load to 500 pli due to shell thickness limitations, and as being limited in roll diameter. These authors state that shoe length is typically ten inches for the impulse drying unit. The line loads disclosed are 1000 pli to 10,000 pli. As a result, this application teaches away from the combined use of a low line load with a substantial peak pressure.
Contrary to the current state of the art, the present inventors have, quite unexpectedly, found that in the production of absorbent paper products, the use of a steep, sharp pressure gradient and controlled separation when producing absorbent paper can improve dewatering efficiency without adversely affecting product properties. An example of the pressure profile of the new shoe design for absorbent paper production according to the present invention is illustrated in FIG. 3.
The present inventors unexpectedly discovered that good sheet dewatering and appropriate bulk/strength properties for low weight absorbent products could be attained with this pressure optimized shoe press. The optimized pressure conditions can be achieved according to the present invention by shaping the shoe, tilting the shoe in the shoe press, reducing the length of the shoe in the shoe press, and/or tapering the exit side of the shoe. In addition, these conditions can also be achieved by deflecting the pressing blanket from the web carrying foraminous-endless-fabric at a point nearly simultaneous with separation of the foraminous-endless-fabric from the nascent web, thereby reducing rewet. These techniques enable the pressure optimized shoe press according to the present invention to achieve improved dewatering while maintaining bulk with line loads less than about 240 kN/m and peak pressures greater than about 2000 kN/m.sup.2.